Methanol fuel cells

ABSTRACT

A cathode for a liquid fuel cell, which is covered with a film that is a liquid fuel barrier, but is permeable to oxygen, or for a direct methanol fuel cell, which is covered with a film that is a methanol barrier, but is permeable to oxygen. The fuel barrier may be made of the polymer of a macrocyclic compound having, in the same molecule, preferably three substituents that allow polymerization and crosslinking. The methanol barrier may be made of a polymerized porphyrin, wherein the porphyrin is chosen from among non-metallated and metallated porphyrins.

FIELD OF THE INVENTION

[0001] This invention relates to direct methanol fuel cells(hereinafter, briefly, DMFC) which are low temperature, solid polymerelectrolyte, fuel cells directly fed by liquid methanol. Morespecifically, this invention relates to cells in which the methanoldiffusion from the anode to the cathode side across the polymerelectrolyte membrane is prevented or greatly reduced, thereby increasingthe oxygen reduction current in the cell. This invention also relates toelectrodes provided with a coating that constitutes a methanol barrierand to methods for preparing such electrodes.

BACKGROUND OF THE INVENTION

[0002] Electrochemical fuel cells convert fuel and oxidant toelectricity and reaction product. Fluid reactants are supplied to a pairof electrodes, which are in contact with and separated by anelectrolyte. The electrolyte may be a solid or a liquid. In the solidpolymer electrochemical fuel cells the electrodes typically comprise anelectrode substrate and an electrocatalyst layer disposed upon one majorsurface of the electrode substrate. The electrode substrate typicallycomprises a sheet of porous, electrically conductive material, such ascarbon fiber paper or carbon cloth. The layer of electrocatalyst istypically in the form of finely comminuted metal, typically platinum,and is disposed on the surface of the electrode substrate at theinterface with the membrane electrolyte in order to induce the desiredelectrochemical reaction. In a single cell, the electrodes areelectrically coupled to provide a path for conducting electrons betweenthe electrodes through an external load.

[0003] At the anode, the fuel moves through the porous anode substrateand is oxidized at the anode electrocatalyst layer. At the cathode, theoxidant moves through the porous cathode substrate and is reduced at thecathode electrocatalyst layer.

[0004] The catalyzed reaction at the anode produces hydrogen cations(protons) from the fuel supply. The ion-exchange membrane facilitatesthe migration of protons from the anode to the cathode. In addition toconducting protons, the membrane isolates the hydrogen-containinggaseous fuel stream from the oxygen-containing gaseous oxidant stream.At the cathode electrocatalyst layer, oxygen reacts with the protonsthat have crossed the membrane to form water as the reaction product.

[0005] In liquid feed, electrochemical fuel cells, one or more of thereactants is introduced to the electrocatalyst in liquid form. Mostcommonly, methanol is the fuel supplied to the anode (so-called “directmethanol” fuel cells) and oxygen to the cathode. In fuel cells of thistype, the reaction at the anode produces protons, which arise from theoxidation of methanol. An electrocatalyst promotes the methanoloxidation at the anode. The protons formed at the anode electrocatalystmigrate through the electrolyte from the anode to the cathode, and atthe cathode electrocatalyst layer the oxidant reacts with the protons toform water.

[0006] In electrochemical fuel cells employing liquid or solidelectrolytes and gaseous or liquid reactant streams, crossover of areactant from one electrode to the other is generally undesirable.Reactant crossover may occur if the electrolyte is permeable to thereactant, that is, some of a reactant introduced at a first electrode ofthe fuel cell may pass through the electrolyte to the second electrode,instead of reacting at the first electrode. Reactant crossover typicallycauses a decrease in both reactant utilization efficiency and fuel cellperformance. Fuel cell performance is defined as the voltage output fromthe cell at a given current density or vice versa; the higher thevoltage at a given current density or the higher the current density ata given voltage, the better the performance.

[0007] In solid polymer, electrochemical fuel cells, the ion-exchangemembrane may be permeable to one or more of the reactants. For example,ion-exchange membranes typically employed in solid polymerelectrochemical fuel cells are permeable to methanol; thus, methanolwhich contacts the membrane prior to participating in the oxidationreaction can cross over to the cathode side. Diffusion of methanol fuelfrom the anode to the cathode leads to a reduction in fuel utilizationefficiency and to performance losses (see, for example, S. Surampudi etal., Journal of Power Sources, Vol. 47, 377-385 (1994) and C. Pu et al.,Journal of the Electrochemical Society, Vol. 142, L119-120 (1995)).

[0008] Fuel utilization efficiency losses arise from methanol diffusionaway from the anode because some of the methanol which would otherwiseparticipate in the oxidation reaction at the anode and supply electronsto do work through the external circuit is lost. Methanol arriving atthe cathode may be lost through vaporization into the oxidant stream, ormay be oxidized at the cathode electrocatalyst, consuming oxidant.

[0009] Methanol diffusion to the cathode may lead to a decrease in fuelcell performance. The oxidation of methanol at the cathode reduces theconcentration of oxygen at the electrocatalyst and may affect access ofthe oxidant to the electrocatalyst. Further, depending upon the natureof the cathode electrocatalyst and the oxidant supply, theelectrocatalyst may be poisoned by methanol oxidation products, orsintered by the methanol oxidation reaction. Diffusion of methanolacross the H⁺-conducting porous polymer electrolyte membrane is one ofthe fundamental problems of the DMFC. To overcome it, it has beenproposed to use barrier layers of electrolyte in the electrolytemembranes, particularly palladium layers.

[0010] It has also been proposed to provide improved polymer electrolytemembranes for DMFCs, utilizing cross-linked polystyrene sulfonic acidwithin electrochemically inert matrices of poly(vinylidene fluoride) orusing other matrix membranes in place of polyvinylene fluoride orblended or co-polymerized with it.

[0011] U.S. Pat. No. 5,874,182 proposes a liquid feed electrochemicalfuel cell comprising: a) a first electrode comprising a quantity ofcatalysts and a self-supporting porous sheet material having first andsecond appositely facing measured surfaces, said first electrode fluidlyconnected to a source of liquid reactant;

[0012] b) a second electrode;

[0013] c) an ion-exchange membrane interposed between said electrodes;wherein said catalyst is distributed through the thickness of said sheetmaterial between said measured surfaces.

[0014] However, the solutions of the prior art are not fullysatisfactory. Palladium is an expensive material and its use as abarrier layer involves lack of stability due to adhesion and oxidationproblems. The membranes proposed in the prior art have lack ofuniformity and decreased proton conductivity. At any rate, all theproposed modifications of the fuel cells do not result in significantreduction of methanol crossover, and the problem of methanol crossoveris still significantly felt.

[0015] A general structure of fuel cells with which this invention isconcerned is schematically indicated in FIG. 1. It comprises a cathode10 and an anode 11 separated by a polymer electrolyte 12. Liquid fuel,in this case methanol, is supplied from a tank 13 to the anode andoxygen (or air) is supplied by a compressor 14 to the cathode. CO₂ isdischarged from tank 13, as indicated at 15. The polymer electrolyte ispermeable to the methanol, which therefore can migrate through thepolymer electrolyte to the cathode.

[0016] It is therefore a purpose of this invention to prevent methanol,and in general, liquid fuel, from coming into contact with the cathode,thereby poisoning the same.

[0017] It is another purpose of this invention to prevent the crossoverof methanol without changing the polymer electrolyte.

[0018] It is a further purpose of this invention to provide a cathodeprovided with a methanol barrier that protects said cathode from beingpoisoned by the methanol.

[0019] It is a still further purpose of the invention to provide such anelectrode whether its the surface is smooth or not.

[0020] It is a still further purpose to provide an improvement in fuelcell electrodes that is efficient even in H₂/O₂ fuel cells wherein thefuel is not methanol.

[0021] It is a still further purpose of the invention to permit todispense, in certain cases, from the presence of the solid electrolyte.

[0022] It is a still further purpose of the invention to provide aprocess for producing said improved electrodes.

[0023] It is a still further purpose of the invention to achieve theaforesaid purposes without significantly hindering the oxygen reductionprocess if the cathode is an oxygen cathode.

[0024] It is a still further purpose of the invention to provide aprocess whereby commercially available catalytic electrodes, which canbe used as oxygen cathodes in fuel cells, are improved to achieve thepurposes of the invention.

[0025] Other purposes and advantages of the invention will appear as thedescription proceeds.

SUMMARY OF THE INVENTION

[0026] According to the invention, in a fuel cell, particularly but notexclusively a direct methanol fuel cell (DMFC), the cathode is coveredwith a film that is a liquid fuel barrier, particularly a methanolbarrier, but is permeable to oxygen. Hereinafter, reference will be madeonly to methanol, for descriptive purpose, but this should not beconstrued as a limitation, since the invention can be applied, mutatismutandis, to cells in which the fuel is different from methanol.

[0027] In a preferred form of the invention, the methanol barrier ismade of a polymerized porphyrin. Preferably, the porphyrin shouldinclude groups that allow the polymerization of the porphyrin; and morepreferably, at least three such groups, not necessarily identical,should be present in the same porphyrin molecule. Compounds that are notporphyrins also come within the scope of the invention, provided thatthey are macrocyclic compounds preferably containing, in the samemolecule, three substituents that allow polymerization and crosslinking.Such compounds yield cross-linked polymers, which are preferredaccording to the invention.

[0028] As has been said, the invention includes fuel cells in which thefuel is not methanol. The fuel could consist of other alcohols, and ingeneral, of any compound the molecule of which can be oxidized at theanode and is large at least as that of methanol.

[0029] In a particular embodiment of the invention, the cathode surfaceis smooth and the barrier film is applied directly onto it. In anotherembodiment of the invention, the cathode surface is not smooth, e.g., itis relatively coarse as in commercially available catalytic electrodes,and an intermediate layer is provided on the surface of the cathode tosmoothen the said surface, whereby the fuel barrier film can be applied,as a second or outer layer, over said intermediate layer and will serveas a methanol barrier.

[0030] The invention also provides a process for generating over theelectrode a coating, which constitutes a fuel barrier, particularly amethanol barrier, by electrochemical polymerization, viz. by applying tothe solution of porphyrin in a solvent a potential lower than that atwhich the solvent significantly decomposes through oxidation at theelectrode surface. While maintaining said upper limit, the potential canbe applied as a constant potential close to said decomposition potentialor lower, or can be cyclically changed or stepped, particularly fromzero to close to said decomposition potential. The solvent can be anacidic aqueous solution, such as e.g. a solution of H₂SO₄ or HClO₄, or abasic aqueous solution, such as a solution of NaOH or KOH, and in thatcase the maximum potential can preferably be in the range +1 to +1.2volts (vs. SCE). The solvent can also be organic, such as e.g.acetonitrile or methylene chloride, and in this case the maximumpotential can be higher.

[0031] According to the invention, the porphyrin or other compound ashereinbefore defined is originally present as a monomer in the solution.The polymerization occurs in the cell. Information as to thepolymerization of porphyrins can be found in A. Bettelheim et al.,Electrochemical Polymerization of Amino-, Pyrrole-, andHydroxy-substituted Tetraphenylporphyrins, Inorg. Chem., 26, 1009-1017,and in A. Bettelheim et al., Electrocatalysis of Dioxygen Reduction inAqueous Acid and Base by Multimolecular Layer Films ofElectropolymerized Cobalt Tetra(o-aninophenyl)porphyrin, J. Electroanal.Chem., 217 (1987) 271-286. A preferred way of carrying out thepolymerization consists in applying a constant potential and maintainingit until the current decreases to a constant and low current. In thisway dense films of polymerized, and preferably cross-linked, porphyrinsare obtained.

[0032] If the electrode has a coarse surface, an intermediate layer isapplied, as stated above, and it should be thick enough to smoothen saidsurface and be of a polymer which is also a good proton conductor. Saidpolymer should have the properties of electric charge conductivity andproton conductivity. Electric charge conductivity is provided bypolymers such as e.g. polyaniline, polypyrrole and polythiophene. Thesulfonic group is one of the best proton conductive groups. Thereforethe said polymers should be preferably chemically bound or doped withthe sulfonic group, or another proton conductive group, by binding thegroup to the corresponding monomer (e.g., aniline, pyrrole or thiophene)or introducing a compound which contains the group as the counter ionduring the electropolymerization of the monomer.

[0033] In another aspect thereof, the invention comprises the use ofpolymerized porphyrins for forming methanol barriers in direct methanolfuel cell,

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In the drawings:

[0035]FIG. 1 is a schematic illustration of the typical two-electrode,liquid fuel cell;

[0036]FIG. 2 illustrates the chemical formula of some porphyrins;

[0037]FIG. 3 shows chronoamperometric curves obtained for oxygenreduction relating to an embodiment of the invention;

[0038]FIGS. 4A and B show oxygen reduction polarization curves relatingto another embodiment of the invention;

[0039] FIGS. 5 to 7 show other polarization curves obtained underconditions and for embodiments described hereinafter;

[0040]FIG. 8 shows the current/potential curves relating to anotherembodiment of the invention; and

[0041]FIG. 9 is a scheme of the three-electrode cell.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0042] As stated hereinbefore, the preferred chemicals for making themethanol barrier on the cathode are porphyrins, particularly chosen fromamong amino-, hydroxy-, pyrrole- and vinyl-substituted porphyrins. Theporphyrins may contain metals in the substituting groups. Metal ion maybe contained in the central porphyrin location, as occurs in certainnatural substances such as heme. The general formula of the saidporphyrin, in which X indicates a wide range of substituents, is shownin FIG. 2. Some substituents X, that form different porphyrins adaptedfor use in this invention, are indicated on FIG. 2, which also indicatesthe abbreviations used to define the various porphyrins defined by saidsubstituents. By way of example, said porphyrins may be chosen fromamong tetrakis(ortho-aminophenyl)porphyrin,

[0043] tetrakis(metha-aminophenyl)porphyrin,

[0044] tetrakis(para-aminophenyl)porphyrin,

[0045] tetrakis(para-dimethylaminophenyl)porphyrin,

[0046] tetrakis(para-hydroxophenyl)porphyrin, and

[0047] tetrakis(para-pyrrolephenyl)porphyrin.

[0048]FIG. 2 and the foregoing list, however, are illustrative but notlimitative: other porphyrins can be used in this invention, such asvinyl substituted porphyrins, e.g. protoporphyrin IX.

[0049] In most of the following examples, three-electrode cells areconsidered. The three-electrode cells comprise a working electrode, acounter electrode and a reference electrode. A scheme of thethree-electrode cell is shown in FIG. 9, which is self-explanatory (seeA. J. Bard and L. R. Faulkner, “Electrochemical Methods, Fundamentalsand Applications”, John Wiley, 1980).

EXAMPLE 1

[0050] Experimental set-up: The experiment was conducted in aconventional three-electrode cell (half-cell configuration). One of twoglassy carbon electrodes cast in Teflon and supplied byMetrohm(Switzerland) (A=0.07 cm²) (abbreviated: A and B) served asworking electrode, a Pt wire in a separate compartment served as counterelectrode and a saturated calomel electrode (SCE) as referenceelectrode. Potential was applied between the working and referenceelectrodes using a 273 potentiostat (EG&G).

[0051] Treatment and coating of working electrodes: Electrodes A and Bwere first polished with a water emulsion of 0.3μ alumina and thencoated with Pt (as an oxygen reduction catalyst) by applying a currentof 5 mA/cm² for 10 minutes from an 10⁻² M H₂PtCl₆+1 M H₂SO₄ solution atroom temperature. Electrode A was then coated with an electropolymerizedfilm of (o-NH₂)TPP (abbreviated: poly(o-NH₂)TPP) by cycling thepotential 20 cycles from 0 to +1.2 V at a scan rate of 50 mV/s from asolution of H₂(o-NH₂)TPP dissolved in 1 M H₂SO₄ (1 mg/ml) at roomtemperature. Electrode B was not coated with this film.

[0052] Results: FIG. 3 shows chronoamperometric curves obtained foroxygen reduction at room temperature at a potential of +0.2 V in asolution of 1M H₂SO₄+1M methanol continuously bubbled with air (100cc/min): curves (a) and (b) for electrodes A and B, as workingelectrode, respectively. It can be seen from the figure that when theelectrode is coated with the methanol barrier film (as in electrode A),the steady oxygen reduction current at +0.2 V and in the presence of 1Mmethanol is −105 μA/cm². However, in the absence of this film (as inelectrode B), the steady oxygen reduction current is only −0.33 μA/cm².This means that the methanol barrier film causes less methanol to reachthe electrode and thus less catalytic platinum catalytic sites arepoisoned by methanol and are therefore available for oxygen reduction.

EXAMPLE 2

[0053] Experimental set-up: The same experimental set-up (half-cellconfiguration) as described in Example 1 was used.

[0054] Treatment and coating of working electrodes: The glassy carbonelectrodes A and B were polished and then coated with Pt as inExample 1. Electrode A was then coated with poly(o-NH₂)TPP by steppingthe potential 30 times from 0 (10 s) to +1.2 V (60 s) from a solution ofH₂(o-NH₂)TPP dissolved in 1M H₂SO₄ (1 mg/ml) at 60° C. Electrode B wasnot coated with this film.

[0055] Results: FIG. 4A shows oxygen reduction polarization curves (at ascan rate of 5 mV/s) obtained in 1M H₂SO₄ solution at 70° C.continuously bubbled with air (100 cc/min) for electrode A in theabsence (curve a) and presence of 0.5 M methanol (curve b).

[0056]FIG. 4B shows oxygen reduction polarization curves (5 mV/s)obtained in the same solution and conditions as in FIG. 3 for electrodeB in the absence (curve a) and presence of 0.5 M methanol (curve b).

[0057] Table I summarizes the currents obtained at two potentials (vs.SCE) for electrodes A and B. TABLE I Oxygen reduction currents obtainedfrom polarization curves (FIGS. 4A and 4B) in 1M H₂SO₄ at 70° C. forelectrodes A and B (Example 2). Current density* Current density*(mA/cm²) (mA/cm²) Electrode at +0.1 V at +0.5 V Electrode A, −0.58 −0.14absence of methanol. Electrode A, presence of −0.56 +0.01 0.5 Mmethanol. Electrode B, −0.42 −0.31 absence of methanol. Electrode B,presence of −0.23 +19.7 0.5 M methanol.

[0058] It can be seen from FIGS. 4A and 4B and from Table I that whileonly 55% of the current for oxygen reduction at +0.1 V remains after theaddition of methanol for electrode B, which is uncoated with a methanolbarrier film, 96% of the current remains at the same conditions for theelectrode coated with the methanol barrier film (electrode A). The factthat almost no methanol crosses the barrier film can also be concludedfrom the very low oxidation current observed after the addition ofmethanol at +0.5 V (originating from methanol oxidation) for theelectrode coated with the barrier film, compared to the very highcurrent observed for the non-coated electrode (0.01 and 19.7 mA/cm², forelectrodes A and B, respectively).

EXAMPLE 3

[0059] Experiments with commercially available cathodes and anodes forfuel cells were conducted first in a three-electrode (half cell) andthen with a two-electrode (full cell) configuration set-up. Example 3describes an experiment conducted with a commercially available cathode.In the two-electrode experiment, the cell used was not as illustrated inFIG. 1: a solution of acid was used as the electrolyte instead of asolid polymer membrane, and the same setup was used as for thethree-electrode experiment setup, but the counter and referenceelectrodes were short-circuited.

[0060] Half-cell experimental set-up: The three-electrode set-up wassimilar to that of experiments 1 and 2. However, the working electrodeused in this experiment was a commercially available electrode suppliedby E-TEK: EFCG “S” Type Electrode on TGFH-120 Toray Carbon Paper, 10%Pt/C, 0.5 mg/cm² Pt loading. Part of the electrode was brushed withmasking paint so that only 1 cm² was exposed to the electrolyticsolution.

[0061] Treatment and coating of working electrode: The reactivity of theabove electrode towards the cathodic reduction of oxygen in a 1M H₂SO₄solution at 60° C. and continuously bubbled with air is low. This can bededuced by the polarization curve A (obtained at a scan rate of 1 mV/s)in FIG. 5. However, we found that after applying a potential of +1.1 V(between this electrode and the reference electrode SCE) in the samesolution and temperature as indicated above, the polarization curvesobtained for oxygen reductions show higher currents. This is shown inFIG. 5 for different times of treatment at +1.1 V (curves B, C, D and Eare for ⅓, 1, 2 and 3 hours treatment, respectively). After a three-hourtreatment, the oxygen reduction current at +0.15 V increased from 0.18to 1.6 mA/cm₂, i.e., an 8.9-fold increase. We attribute this effect asresulting from the evolvement of oxygen due to electrochemical oxidationof water at +1.1 V. The gas evolved in narrow pores contained in theelectrode structure probably causes the increase of the catalyticeffective area, a process that we call: “electrochemical digging”.

[0062] The second treatment step was to coat with a thick polymeric filmwhich smoothens the surface morphology. The polymer which we found thatis suitable for this purpose is poly(aniline sulfonic acid). Thispolymer also is a good proton conductor (due to the presence of thesulfonic groups), a necessary property of electrodes in fuel cells.Coating of the electrode with this polymer was conductedelectrochemically (electropolymerization) by cycling the potentialapplied on the electrode between 0 and +0.8 V (scan rate of 50 mV/s, 60cycles) in a solution containing 1M H₂SO₄ and 5×10⁻² M monomeric anilinesulfonic acid at a temperature of 60° C. FIG. 6 shows the polarizationoxygen reduction curve obtained (same solution and temperature as inFIG. 5) after this coating procedure, compared to that obtained afterthe “electrochemical digging” procedure (curves B and A, respectively).Further coating of poly(aniline sulfonic acid) on the same electrode wasachieved by applying a constant potential of +0.8 V for 30 minutes inthe H₂SO₄+aniline sulfonic acid solution (60° C.). Curve C representsthe polarization reduction curve for oxygen after this step. From FIG.6, it can be deduced that the polymeric coating does not perturb oxygendiffusion to the catalytic layer. Moreover, this coating even increasesthe electrode reactivity towards oxygen reduction, probably due to theintroduction of proton conductive sulfonic acid groups into thecatalytic layer. Continuation of the coating process, however, causesdecrease of the oxygen reduction current (probably due to decrease ofoxygen permeability onto very thick polymeric films).

[0063] The last treatment step was to coat a layer of poly(o-NH₂)TPP ontop of the poly(aniline sulfonic acid) layer by applying a constantpotential of +1.05 V from a solution of H₂(o-NH₂)TPP dissolved in 1MH₂SO₄ (1 mg/ml) at 60° C. Polarization curves were recorded as after theprevious treatments and the currents obtained at +0.05 V before andafter the addition of 1% methanol in solution are summarized in Table IIfor increasing times of H₂(o-NH₂)TPP electropolymerization. TABLE IIOxygen reduction currents obtained in the absence and presence of 1%methanol in 1 M H₂SO₄ at 60° C. after increasing H₂(o-NH₂)TPPelectropolymerization time. Electropolymerization Current densityCurrent density Ratio of currents time (hours) for (mA/cm²) at +0.05 V(mA/cm²) at +0.05 V after and before H₂(o-NH₂)TPP without methanol with1% methanol adding methanol (%) 2 3.95 3.29 83 5 4.38 3.93 89.7 9 3.463.13 90.5

[0064] From Table II, it can be deduced that increasing theelectropolymerization time from 2 to 5 hours increases the oxygenreduction current obtained in the absence of methanol. Moreover, lessdecrease of the current is observed after addition of methanol whichindicates that less methanol crosses the coating when increasing theelectropolymerization time from 2 to 5 hours. A further increase from 5to 9 hours causes decrease of the current in the absence of methanolwith only a small contribution to the ratio of currents after and beforethe methanol addition (currents ratio increase from 89.7 to 90.5%).

[0065] The performance of the electrode with its special treatment andcoating that we have developed towards oxygen reduction in the presenceof methanol was compared to that of a similar electrode which has beentreated by “electrochemical digging”, but which has not been coated withpoly(aniline sulfonic acid) and poly(o-NH₂)TPP. Instead, this electrodewas treated with a conventional Nafion coating (dipping for a half hourin a 5% Nafion solution). FIG. 7 shows the oxygen reduction polarizationcurves obtained in 1 M H₂SO₄+2% methanol at 60° C. (air flow rate: 100cc/min) for the Nafion treated electrode (curve A), compared to theelectrode coated with poly(aniline sulfonic acid) and poly(o-NH₂)TPP(curve B). It can be clearly seen from this figure that methanol poisonsthe Nafion coated electrode much more than the electrode with thecoating that we developed. This can be deduced by the high anodiccurrents observed at high positive potentials for the Nafion coatedelectrode (+2 mA/cm² at +0.5 V, compared to −0.03 mA/cm² at the samepotential for the other electrode) as well by the higher cathodiccurrents for oxygen reduction obtained for the electrode with thecoating that we developed (−3.7 mA/cm² at +0.05 V, compared to −1.4mA/cm² at the same potential for the Nafion coated electrode).

EXAMPLE 4

[0066] Example 4 describes an experiment conducted with a commerciallyavailable cathode and anode in a two-electrode (full-cell)configuration.

[0067] Full-cell experimental set-up: The cathode used in thisexperiment was the same electrode used in Example 3, supplied by E-TEK(EFCG “S” Type Electrode on TGFH-120 Toray Carbon Paper, 10% Pt/C, 0.5mg/cm² Pt loading). This electrode was first treated by “electrochemicaldigging” and then coated with poly(aniline sulfonic acid) and thenpoly(o-NH₂)TPP, as described in Example 3. The anode, also supplied byE-TEK, was an EFCG electrode: TGPH-120 Toray Carbon Paper, 10% Pt/Ru/C(1:1 a/o), 0.5 mg/cm² Pt/Ru. Part of the electrode was brushed withmasking paint so that only 1 cm² was exposed to the electrolyticsolution. The two electrodes were introduced in a solution of 1 M H₂SO₄at 60° C., bubbled continuously with air (100 cc/min) and the cathodeand anode reactions were driven by a potentiostat (EG&G, model 273).FIG. 8 shows the current/potential curves obtained (at a scan rate of 1mV/s) in the presence of 1 and 2% methanol (curves A and B,respectively). The figure also shows the current/potential curveobtained in 1 M H₂SO₄+2% methanol at 60° C. when air is replaced byoxygen (curve C). This figure demonstrates the concept we developed: itis possible to operate a fuel cell without using a membrane separatingthe cathode from the anode if the cathode is coated with a special thinfilm which prevents (or reduces) methanol to permeate through thecatalytic layer of the cathode. The low currents obtained in thisresearch stage can be increased by using the cathode as a gas diffusionelectrode, a property which has not yet been exploited. Moreover, use ofanodes with a smaller Pt content will also improve the fuel cellperformance, since oxygen reaching the anode is reduced at thiselectrode and therefore reduces the overall current.

[0068] It has been found that in some cases the presence of the methanolbarrier film on the cathode permits to omit the presence of theelectrolyte membrane in the cell, thus simplifying its construction.This is particularly true in applications employing low currentdensities, e.g. less than 10 mA/cm².

[0069] While examples have been given by way of illustration ofembodiments, it will be apparent that the invention is not limited tothem, and can be carried into practice with many modifications,variations and adaptations, without departing from its spirit orexceeding the scope of the claims.

1. A cathode for a liquid fuel cell, which is covered with a film thatis a liquid fuel barrier, but is permeable to oxygen.
 2. A cathodeaccording to claim 1, for a direct methanol fuel cell, which is coveredwith a film that is a methanol barrier, but is permeable to oxygen.
 3. Acathode according to claim 1, wherein the fuel barrier is made of thepolymer of a macrocyclic compound having, in the same molecule,preferably three substituents that allow polymerization andcrosslinking.
 4. A cathode according to claim 2, wherein the methanolbarrier is made of a polymerized porphyrin, wherein the porphyrin ischosen from among non-metallated and metallated porphyrins.
 5. A cathodeaccording to claim 4, wherein the polymerized porphyrin is cross-linked.6. A cathode according to claim 1, wherein the cathode surface is smoothand the barrier film is applied directly onto it.
 7. A cathode accordingto claim 1, wherein the cathode surface is not smooth, an intermediatelayer is provided on the surface of the cathode to smoothen the saidsurface, and the fuel barrier film is applied, as an outer layer, overthe intermediate layer.
 8. A cathode according to claim 5, wherein theintermediate layer is made of a polymer which is electrically conductiveand is a good proton conductor.
 9. A cathode according to claim 6,wherein the intermediate layer is made of a polymer chosen from amongpolyaniline, polypyrrole and polythiophene, chemically bound or dopedwith sulfonate or other proton conductive groups.
 10. A cathodeaccording to claim 3, wherein the porphyrin is chosen from among:tetrakis(ortho-aminophenyl)porphyrin,tetrakis(metha-aminophenyl)porphyrin,tetrakis(para-aminophenyl)porphyrin,tetrakis(para-dimethylaminophenyl)porphyrin,tetrakis(para-hydroxophenyl)porphyrin,tetrakis(para-pyrrolephenyl)porphyrin, Protoporphyrin IX.
 11. Processfor applying a coating for making a cathode according to claim 4, whichcomprises dissolving the porphyrin in a suitable solvent andpolymerizing said porphyrin by applying a potential lower than that atwhich the solvent significantly decomposes through oxidation at theelectrode surface.
 12. Process according to claim 11, wherein thepotential is kept at a constant value preferably in the range +1 to +1.2volts (vs. SCE).
 13. Process according to claim 11, wherein thepotential is cyclically varied from zero to a potential close to that atwhich the solvent decomposes through oxidation at the electrode surface.14. Process according to claim 11, wherein the potential is cyclicallyvaried from zero to about +1.2 volts.
 15. Process according to claim 11,wherein the solvent is chosen from among aqueous solutions of acids andbases and organic solvents.
 16. Process according to claim 15, whereinthe acid is chosen from among H₂SO₄ and HClO₄, the base is chosen amongNaOH and KOH, and the organic solvent is chosen from among acetonitrileand methylene chloride.
 17. Process for improving the reactivity of anelectrode towards the reduction of oxygen, which comprises applying tothe electrode a potential of +1.1 V in a 1M H₂SO₄ solution and at atemperature of 60° C. for a time of 1 or a few hours.
 18. Liquid fuelcell, comprising an anode, a solid electrolyte and a cathode accordingto claim
 1. 19. Direct methanol fuel cell, comprising an anode, a solidelectrolyte and a cathode according to claim
 2. 20. Direct methanol fuelcell, comprising an anode and a cathode according to claim 2, in theabsence of a solid electrolyte.